Semiconductor storage device and method of manufacturing the same

ABSTRACT

A method of manufacturing a semiconductor storage device according to an embodiment of the present invention includes forming dummy cells 61 1 , to 61 8  at a position adjacent to a reference cell 41 2 , and implanting an impurity into the dummy cells 61 1 , to 61 8  using a mask that covers the reference cell 41 2 . Here, the process of implanting the impurity is carried out so that the impurity exudes out of the dummy cells 61 1 , to 61 8  to the reference cell 41 2 .

This application is based on Japanese patent application No. 2005-237235, the content of which is incorporated hereinto by reference.

BACKGROUND

1. Technical Field

The present invention relates to a semiconductor storage device and a method of manufacturing the same.

2. Related Art

A sense amp included in a semiconductor storage device is a circuit that quickly detects a minute fluctuation in status of each memory cell (such as fluctuation in current and potential) in a form of a large potential amplitude. Characteristics required from the sense amp include high speed, a wide range of voltage and temperature, and a sufficient margin for process fluctuation and power source noise. Now that the ultra high level of integration progresses, the range of voltage and temperature, as well as the margin for process fluctuation are being forced to be reduced, and hence such characteristics have to be improved.

The semiconductor storage devices so far developed include the one disclosed in Japanese patent publication No. 3578661. The semiconductor storage device according to this document is a non-volatile semiconductor storage device such as a mask ROM, which retains data stored in the memory cell despite turning off. This device includes, as shown in FIG. 4, a memory cell 111 ₁, selection cells 112 ₁₁, 112 ₂₁, a word line 113, column lines 114 ₁, 114 ₂, a word line drive circuit 115, column selection circuits 116 ₁, 116 ₂, a read-out driver 119 ₁, a buffer 120 ₁, a reference unit 122, and a sense amp 123 ₁.

The memory cell 111 ₁ is constituted of MOS transistors, the gates of which are mutually connected via the word line 113, and are connected to an output terminal of the word line drive circuit 115 via the word line 113.

The selection cell 112 ₁₁ is constituted of MOS transistors, the gates of which are mutually connected via the column line 114 ₁, and are connected to an output terminal of the column selection circuit 116 ₁ via the column line 114 ₁. The column selection circuit 116 ₁ decodes an address provided from outside at a first stage decoding and applies, when the column line 114 ₁ is thereby selected, a high level (hereinafter, H-level) signal to the column line 114 ₁. Accordingly, the H-level signal is applied to the gate of the selection cell 112 ₁₁, so that the selection cell 112 ₁₁ is turned ON thus to form a path for reading out therethrough the data from the memory cell 111 ₁.

The selection cell 112 ₂₁ is constituted of MOS transistors, the gates of which are mutually connected via the column line 114 ₂, and are connected to an output terminal of the column selection circuit 116 ₂ via the column line 114 ₂. The column selection circuit 116 ₂ decodes an address provided from outside at a second stage decoding and applies, when the column line 114 ₂ is thereby selected, a H-level signal to the column line 114 ₂. Accordingly, the H-level signal is applied to the gate of the selection cell 112 ₂₁, so that the selection cell 112 ₂₁ is turned ON thus to form a path for reading out therethrough the data from the memory cell 111 ₁.

Once a signal instructing to read out the data is received from outside, a low level (hereinafter, L-level) signal indicating start of the data read-out is applied to a sense amp activating signal, which provides the L-level signal to the read-out driver 119 ₁, the buffer 120 ₁ and the reference unit 122.

The read-out driver 119 ₁ includes a drive transistor 131 ₁, a path-forming transistor 132 ₁, a path-blocking transistor 133 ₁, and a NOR gate 134 ₁.

The drive transistor 131 ₁ is constituted of a MOS transistor, and applies a voltage according to the ON/OFF status of the memory cell 111 ₁ to an input terminal of the buffer 120 ₁. The path-forming transistor 132 ₁ is constituted of a MOS transistor, and is turned ON by a H-level signal provided by the NOR gate 134 ₁, so as to form a path for reading out the data therethrough from the memory cell 111 ₁. The path-blocking transistor 133 ₁ is constituted of a MOS transistor, and is turned ON by a H-signal provided by the sense amp activating signal, so as to block the path for reading out the data therethrough from the memory cell 111 ₁. The NOR gate 134 ₁ receives the sense amp activating signal at a first input terminal thereof. The NOR gate 134 ₁ has a second input terminal connected to the source of the path-forming transistor 132 ₁, so as to output a H-level signal to turn ON the path-forming transistor 132 ₁, when the signal provided by the sense amp activating signal and the voltage of the source of the path-forming transistor 132 ₁ are both at the L-level.

The buffer 120 ₁ includes power MOS transistors 135 ₁, 136 ₁, and a MOS transistor 137 ₁ that constitutes a constant current source. The buffer 120 ₁ serves to buffer and amplify an input voltage, and to apply an output voltage V_(D1) thereof to a first input terminal of the sense amp 123 ₁.

The reference unit 122 includes reference cells 141 ₁, 141 ₂, selection cells 142 ₁₁, 142 ₁₂, 142 ₂₁, 142 ₂₂, a word line drive circuit 143, column selection circuits 144 ₁, 144 ₂, drive transistors 145 ₁, 145 ₂, path-forming transistors 146 ₁, 146 ₂, path-blocking transistors 147 ₁, 147 ₂, NOR gates 148 ₁, 148 ₂, power MOS transistors 149 ₁, 149 ₂, 150 ₁, 150 ₂, and MOS transistors 151 ₁, 151 ₂ that constitute a constant current source.

The reference cells 141 ₁, 141 ₂ are MOS transistors which have the same structure and characteristic with the memory cell 111 ₁. The reference cell 141 ₁ is set in advance as a depression transistor, namely in an ON state, by phosphor ion implantation. The other reference cell 141 ₂ is set in advance as an enhance transistor, namely in an OFF state, without undergoing the phosphor ion implantation.

The selection cells 142 ₁₁, 142 ₁₂ have the same structure and characteristic with the selection cell 112 ₁₁; the selection cells 142 ₂₁, 142 ₂₂ with the selection cell 112 ₂₁; the word line drive circuit 143 with the word line drive circuit 115; the column selection circuit 144 ₁ with the column selection circuit 116 ₁; the column selection circuit 144 ₂ with the column selection circuit 116 ₂; the drive transistors 145 ₁, 145 ₂ with the drive transistor 131 ₁; and the path-forming transistors 146 ₁, 146 ₂ with the path-forming transistors 132 ₁, respectively.

Likewise, path-blocking transistor 147 ₁, 147 ₂ have the same structure and characteristic with the path-blocking transistor 133 ₁; the NOR gates 148 ₁, 148 ₂ with the NOR gate 134 ₁; the power MOS transistors 149 ₁, 149 ₂, 150 ₁, 150 ₂ with the power MOS transistors 135 ₁, 136 ₁; and the MOS transistors 151 ₁, 151 ₂ with the MOS transistor 137 ₁, respectively.

A purpose of such configuration is, because the sense amp 123 ₁ is constituted of a differential amplifier, to equilibrate as much as possible a load connected to the first input terminal thereof with a load connected to the second input terminal thereof.

The drive transistor 145 ₁ applies a voltage V_(RON) according to an ON state of the reference cell 141 ₁ to the gate of the power MOS transistor 150 ₁. Likewise, the drive transistor 145 ₂ applies a voltage V_(ROFF) according to an OFF state of the reference cell 141 ₂ to the gate of the power MOS transistor 150 ₂.

The power MOS transistors 149 ₁, 150 ₁, and the MOS transistor 151 ₁ constituting the constant current source buffer and amplify the output voltage V_(RON) of the drive transistor 145 ₁. On the other hand, the power MOS transistors 149 ₂, 150 ₂, and the MOS transistor 151 ₂ constituting the constant current source buffer and amplify the output voltage V_(ROFF) of the drive transistor 145 ₂. That is, the power MOS transistors 149 ₁, 149 ₂, 150 ₁, 150 ₂ and the MOS transistors 151 ₁, 151 ₂ constitute a buffer 152.

Accordingly, when an output current of the buffer of the power MOS transistor 150 ₁ is denoted by I₁, and an output current of the power MOS transistor 150 ₂ by I₂, a current I_(R), which is the average of the current I₁ and the current I₂ as indicated by the equation (1), runs through the constant current sources, respectively constituted of the MOS transistor 151 ₁ and 151 ₂. I _(R)=(I ₁ +I ₂)/2  (1)

To the second input terminal of the sense amp 123 ₁, therefore, a voltage V_(R), which is the average of the voltage V_(RON) according to the ON state of the reference cell 141 ₁ and the voltage V_(ROFF) according to the OFF state of the reference cell 141 ₂ as indicated by the equation (2), is applied. V _(R)=(V _(RON) +V _(ROFF))/2  (2)

As already stated the sense amp 123 ₁ is constituted of a differential amplifier, so as to detect and amplify a difference between the voltage supplied by the buffer 120 ₁ and the voltage supplied by the reference unit 122, and outputs the data to outside.

SUMMARY OF THE INVENTION

With such configuration, however, reduction in pitch between the respective memory cells because of the progress in the level of integration disables the fabrication of the MOS transistors and memory cells having “the same structure and characteristic”. Referring to FIG. 5, a configuration of a memory cell of a mask ROM will be described as an example.

FIG. 5 is a schematic diagram showing a detailed configuration of the word line drive circuit 115, the memory cell 111 ₁ and the word line 113 shown in FIG. 4. In FIG. 5, single circles represent ON cells, and double circles represent OFF cells. In this example, the memory cell 111 ₁ is an OFF cell.

The ON cells and the OFF cells in the mask ROM are distinguished, as stated above, by whether the cell is subjected to the phosphor ion implantation. A photoresist (PR) with openings at the positions of the ON cells but covering the OFF cells is employed, so that the cells at the openings are subjected to the ion implantation, to thereby turn into depression transistors, namely ON cells. In contrast, the covered cells are not subjected to the ion implantation, thereby turning into enhance transistors, namely OFF cells.

Because of the reduction in pitch between the memory cells due to the progress in level of integration, however, the ion in the ON cells (memory cells 111 ₂ to 111 ₉) may exude toward the adjacent memory cell 111 ₁ which is the OFF cell. This leads to a drop in threshold value of the memory cell 111 ₁, which causes a leak current to run between the drain and the source of the cell, thereby provoking an erroneous decision by the sense amp 123 ₁. Such phenomenon randomly takes place even under the identical ROM arrangement (layout of the ON cells and the OFF cells). Therefore, despite setting the voltage to be applied to the second input terminal of the sense amp 123 ₁ at the median value between the V_(DON) and the V_(DOFF), the erroneous decision may still be committed.

Such aspect will be further described referring to FIG. 6. With the start of the data read-out, the voltage V_(D1) and the voltage V_(R) respectively applied to the first and the second input terminal of the sense amp 123 ₁ increase through a generally similar process, until the selection cells 112 ₁₁, 112 ₂₁, the selection cells 142 ₁₁, 142 ₂₁ and the selection cells 142 ₁₂, 142 ₂₂ are turned ON. Then, until a H-level signal is applied to a location other than the word line 113, the voltage V_(D1) and the voltage V_(R) increase through a similar process (point A in FIG. 6). After a H-level signal is applied to a location other than the word line 113, the voltage V_(R) keeps increasing along an unchanged slope because the word line drive circuit 143 is a dummy circuit, thus to be saturated (line B in FIG. 6).

In contrast, the voltage V_(D1) starts to decrease when the memory cell 111 ₁ is an ON cell (line C in FIG. 6). When the memory cell is an OFF cell and does not incur cell leak, the voltage V_(D1) keeps increasing further (line D in FIG. 6), however, when the memory cell is an OFF cell and incurs cell leak, the voltage V_(D1) settles close to V_(R) (line E in FIG. 6). Accordingly, when the memory cell 111 ₁ is an OFF cell and incurs cell leak, the level of the V_(D1) and the V_(R) is reversed, thus provoking an erroneous decision.

According to the present invention, there is provided a method of manufacturing a semiconductor storage device that compares a voltage between memory cells and reference cells to thereby read out data stored in the memory cells, comprising forming a dummy cell at a position adjacent to a reference cell out of the reference cells that is set in an OFF state; and implanting an impurity into the dummy cell using a mask that covers the reference cell set in an OFF state; wherein the impurity is implanted so as to exude out of the dummy cell to the reference cell set in an OFF state.

By the method of manufacturing thus arranged, the impurity is implanted into the dummy cell adjacent to the reference cell so as to exude out of the dummy cell to the reference cell. This provokes leak from the reference cell, under a high temperature. The leak lowers the voltage level of the reference cell, thereby preventing the voltage of the memory cell and that of the reference cell from being reversed, even when the memory cell incurs leak.

According to the present invention, there is provided a semiconductor storage device comprising a reference cell that includes a first region located between two isolation regions and having a first impurity concentration, and a second region located between the first region and_at least one of the isolation regions and having a second impurity concentration higher than the first impurity concentration.

In the semiconductor storage device thus constructed, the reference cell includes the first region having a relatively low impurity concentration, and the second region having a relative high impurity concentration. Such structure provokes leak from the reference cell, under a high temperature. Therefore, as stated above, the voltage of the memory cell and that of the reference cell can be prevented from being reversed.

The present invention also provides a semiconductor storage device that reads out information stored in a memory cell through comparison with information of a reference cell serving as a standard, wherein the memory cell has a first off-leak characteristic; and the reference cell has a second off-leak characteristic larger than the first off-leak characteristic.

In the semiconductor storage device thus configured, the reference cell has a larger off-leak characteristic than the memory cell. Such configuration provokes leak from the reference cell, under a high temperature. Therefore, as stated above, the voltage of the memory cell and that of the reference cell can be prevented from being reversed.

Thus, the present invention provides a semiconductor storage device in which a probability of an erroneous decision at the time of read-out is lowered, and a method of manufacturing such semiconductor storage device.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, advantages and features of the present invention will be more apparent from the following description taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a circuit diagram of a semiconductor storage device according to an embodiment of the present invention;

FIG. 2 is a schematic diagram showing a detailed configuration of a word line drive circuit, reference cells and a word line in the semiconductor storage device of FIG. 1;

FIG. 3 is a graph for explaining an advantageous effect of a method of manufacturing according to an embodiment;

FIG. 4 is a circuit diagram of a conventional semiconductor storage device;

FIG. 5 is a schematic diagram showing a detailed configuration of a word line drive circuit, memory cells and a word line in the semiconductor storage device of FIG. 4;

FIG. 6 is a graph for explaining a drawback of the conventional semiconductor storage device;

FIG. 7 includes a schematic plan view and cross-sectional view for explaining the method of manufacturing according to the embodiment;

FIG. 8 includes a schematic plan view and cross-sectional view for explaining the method of manufacturing according to the embodiment; and

FIG. 9 is a circuit diagram corresponding to a reference cell array shown in FIGS. 7 and 8.

DETAILED DESCRIPTION

The invention will be now described herein with reference to illustrative embodiments. Those skilled in the art will recognize that many alternative embodiments can be accomplished using the teachings of the present invention and that the invention is not limited to the embodiments illustrated for explanatory purposes.

Hereunder, exemplary embodiments of a semiconductor storage device and a method of manufacturing the same according to the present invention will be described in details, referring to the accompanying drawings. In the drawings, same constituents are given the identical numerals, and duplicating description will not be repeated where appropriate.

FIG. 1 is a circuit diagram of a semiconductor storage device according to an embodiment of the present invention. The semiconductor storage device 1 is a non-volatile semiconductor storage device, and includes a memory cell 11 ₁, selection cells 12 ₁₁, 12 ₂₁, a word line 13, column lines 14 ₁, 14 ₂, a word line drive circuit 15, column selection circuits 16 ₁, 16 ₂, a read-out driver 19 ₁, a buffer 20 ₁, a reference unit 22, and a sense amp 23 ₁.

The memory cell 11 ₁ is constituted of MOS transistors, the gates of which are mutually connected via the word line 13, and are connected to an output terminal of the word line drive circuit 15 via the word line 13.

The selection cell 12 ₁₁ is constituted of MOS transistors, the gates of which are mutually connected via the column line 14 ₁, and are connected to an output terminal of the column selection circuit 16 ₁ via the column line 14 ₁. The column selection circuit 16 ₁ decodes an address provided from outside at the first stage decoding and applies, when the column line 14 ₁ is thereby selected, a H-level signal to the column line 14 ₁. Accordingly, the H-level signal is applied to the gate of the selection cell 12 ₁₁, so that the selection cell 12 ₁₁ is turned ON thus to form a path for reading out therethrough the data from the memory cell 11 ₁.

The selection cell 12 ₂₁ is constituted of MOS transistors, the gates of which are mutually connected via the column line 14 ₂, and are connected to an output terminal of the column selection circuit 16 ₂ via the column line 14 ₂. The column selection circuit 16 ₂ decodes an address provided from outside at the second stage decoding and applies, when the column line 14 ₂ is thereby selected, a H-level signal to the column line 14 ₂. Accordingly, the H-level signal is applied to the gate of the selection cell 12 ₂₁, so that the selection cell 12 ₂₁ is turned ON thus to form a path for reading out therethrough the data from the memory cell 11 ₁.

Once a signal instructing to read out the data is received from outside, a L-level signal indicating start of the data read-out is applied to a sense amp activating signal, which provides the L-level signal to the read-out driver 19 ₁, the buffer 20 ₁ and the reference unit 22.

The read-out driver 19 ₁ includes a drive transistor 31 ₁, a path-forming transistor 32 ₁, a path-blocking transistor 33 ₁, and a NOR gate 34 ₁.

The drive transistor 31 ₁ is constituted of a MOS transistor, and applies a voltage according to the ON/OFF status of the memory cell 11 ₁ to an input terminal of the buffer 20 ₁. The path-forming transistor 32 ₁ is constituted of a MOS transistor, and is turned ON by a H-level signal provided by the NOR gate 34 ₁, so as to form a path for reading out the data therethrough from the memory cell 11 ₁. The path-blocking transistor 33 ₁ is constituted of a MOS transistor, and is turned ON by a H-signal provided by the sense amp activating signal, so as to block the path for reading out the data therethrough from the memory cell 11 ₁. The NOR gate 34 ₁ receives the sense amp activating signal at a first input terminal thereof. The NOR gate 34 ₁ has a second input terminal connected to the source of the path-forming transistor 32 ₁, so as to output a H-level signal to turn ON the path-forming transistor 32 ₁, when the signal provided by the sense amp activating signal and the voltage of the source of the path-forming transistor 32 ₁ are both at the L-level.

The buffer 20 ₁ includes power MOS transistors 35 ₁, 36 ₁, and a MOS transistor 37 ₁ that constitutes a constant current source. The buffer 20 ₁ serves to buffer and amplify an input voltage, and to apply an output voltage V_(D1) thereof to a first input terminal of the sense amp 23 ₁.

The reference unit 22 includes reference cells 41 ₁, 41 ₂, selection cells 42 ₁₁, 42 ₁₂, 42 ₂₁, 42 ₂₂, a word line drive circuit 43, column selection circuits 44 ₁, 44 ₂, drive transistors 45 ₁, 45 ₂, path-forming transistors 46 ₁, 46 ₂, path-blocking transistors 47 ₁, 47 ₂, NOR gates 48 ₁, 48 ₂, power MOS transistors 49 ₁, 49 ₂, 50 ₁, 50 ₂, and MOS transistors 51 ₁, 51 ₂ that constitute a constant current source.

The reference cells 41 ₁, 41 ₂ are MOS transistors which have the same structure and characteristic with the memory cell 11 ₁. The reference cell 41 ₁ is set in advance as a depression transistor, namely in an ON state, by phosphor ion implantation. The other reference cell 41 ₂ is set in advance as an enhance transistor, namely in an OFF state, without undergoing the phosphor ion implantation.

The selection cells 42 ₁₁, 42 ₁₂ have the same structure and characteristic with the selection cell 12 ₁₁; the selection cells 42 ₂₁, 42 ₂₂ with the selection cell 12 ₂₁; the word line drive circuit 43 with the word line drive circuit 15; the column selection circuit 44 ₁ with the column selection circuit 16 ₁; the column selection circuit 44 ₂ with the column selection circuit 16 ₂; the drive transistors 45 ₁, 45 ₂ with the drive transistor 31 ₁; and the path-forming transistors 46 ₁, 46 ₂ with the path-forming transistors 32 ₁, respectively.

Likewise, path-blocking transistors 47 ₁, 47 ₂ have the same structure and characteristic with the path-blocking transistor 33 ₁; the NOR gates 48 ₁, 48 ₂ with the NOR gate 34 ₁; the power MOS transistors 49 ₁, 49 ₂, 50 ₁, 50 ₂ with the power MOS transistors 35 ₁, 36 ₁; and the MOS transistors 51 ₁, 51 ₂ with the MOS transistor 37 ₁, respectively.

A purpose of such configuration is, because the sense amp 23 ₁ is constituted of a differential amplifier, to equilibrate as much as possible a load connected to the first input terminal thereof with a load connected to the second input terminal thereof.

The drive transistor 45 ₁ applies a voltage V_(RON) according to an ON state of the reference cell 41 ₁ to the gate of the power MOS transistor 50 ₁. Likewise, the drive transistor 45 ₂ applies a voltage V_(ROFF) according to an OFF state of the reference cell 41 ₂ to the gate of the power MOS transistor 50 ₂.

The power MOS transistors 49 ₁, 50 ₁, and the MOS transistor 51 constituting the constant current source buffer and amplify the output voltage V_(RON) of the drive transistor 45 ₁. On the other hand, the power MOS transistors 49 ₂, 50 ₂, and the MOS transistor 51 ₂ constituting the constant current source buffer and amplify the output voltage V_(ROFF) of the drive transistor 45 ₂. That is, the power MOS transistors 49 ₁, 49 ₂, 50 ₁, 50 ₂ and the MOS transistors 51 ₁, 51 ₂ constitute a buffer 52.

Accordingly, when an output current of the buffer of the power MOS transistor 50 ₁ is denoted by I₁, and an output current of the power MOS transistor 50 ₂ by I₂, a current I_(R), which is the average of the current I₁ and the current I₂ (Ref. the foregoing equation (1)), runs through the constant current sources, respectively constituted of the MOS transistor 51 ₁ and 51 ₂.

To the second input terminal of the sense amp 23 ₁, therefore, a voltage V_(R), which is the average of the voltage V_(RON) according to the ON state of the reference cell 41 ₁ and the voltage V_(ROFF) according to the OFF state of the reference cell 41 ₂ (Ref. the foregoing equation (2)), is applied.

As already stated the sense amp 23 ₁ is constituted of a differential amplifier, so as to detect and amplify a difference between the voltage supplied by the buffer 20 ₁ and the voltage supplied by the reference unit 22, and outputs the data to outside.

FIG. 2 is a schematic diagram showing a detailed configuration of the word line drive circuit 43, the reference cell 41 ₂ and the word line 53 in the semiconductor storage device 1. In FIG. 2, single circles represent ON cells, and double circles represent OFF cells. In this embodiment, the reference cell 41 ₂ is an OFF cell.

Around the reference cell 41 ₂, dummy cells 61 ₁ to 61 ₈ are disposed. The dummy cells 61 ₁ to 61 ₈ are located adjacent to the reference cell 41 ₂. The output terminal of the word line drive circuit 43 is connected to the gates of the respective cells via the word line 53. The drains of the cells are connected to the source of cells at a lower stage, and the sources of the cells at the lowermost stage are grounded. The drains of the cells at the uppermost stage are connected to the source of the selection cell 42 ₂₂ (FIG. 1).

Hereunder, a method of manufacturing the semiconductor storage device 1 will be described, as an embodiment of a method of manufacturing a semiconductor storage device according to the present invention. The method of manufacturing includes forming the dummy cells 61 ₁ to 61 ₈ at a position adjacent to the reference cell 41 ₂, and implanting an impurity into the dummy cells 61 ₁ to 61 ₁₈ using a mask that covers the reference cell 41 ₂. Here, the process of implanting the impurity is carried out so that the impurity exudes out of the dummy cells 61 ₁ to 61 ₈ to the reference cell 41 ₂. An example of the impurity is phosphor ion.

To be more detailed, openings of the mask corresponding to the dummy cell 61 ₁ to 61 ₈ (indicated by broken lines L1 in FIG. 7) are made larger in area than openings corresponding to those memory cells to be set in an ON state (indicated by broken lines L2 as a reference), among the memory cells, as shown in FIG. 7. In FIG. 7, the upper portion is a plan view of the reference cell array, and the lower portion is a cross-sectional view taken along the line A-A in the plan view. As is apparent from FIG. 7, the word line 53 extends in a direction perpendicular to a direction along which diffusion layers 72 and isolation regions 74 are aligned.

Making the openings corresponding to the dummy cells 61 ₁ to 61 ₈ in a larger size as above allows the impurity implanted through the openings for the dummy cells 61 ₁ to 61 ₈ to reach the diffusion layer of the reference cell 41 ₂ as indicated by the arrow A1. Accordingly, the impurity exudes in a region enclosed by the broken lines C1. As a result of such exudation, the reference cell 41 ₂ obtains a first region located between two isolation regions 74 and having a first impurity concentration, and a second region (where the impurity has exuded) located between the first region and at least one of the isolation regions 74 and having a second impurity concentration higher than the first impurity concentration. Here, the impurity concentration of the dummy cells 61 ₄, 61 ₅ on the respective sides of the reference cell 41 ₂ is equal to or higher than the second impurity concentration. Also, in the semiconductor storage device 1, the memory cell 11 ₁ gains a first off-leak characteristic, and the reference cell 41 ₂ gains a second off-leak characteristic larger than the first off-leak characteristic.

Unlike the above, when the openings for the dummy cells 61 ₁ to 61 ₈ are made in the same size as those for the memory cells, the impurity implanted through the openings for the dummy cells 61 ₁ to 61 ₈ does not reach the diffusion layer of the reference cell 41 ₂ as indicated by the arrow A2, and is hence kept from exuding.

In addition, in the direction along which the diffusion layers 72 and the isolation regions 74 are aligned, the impurity exudes out of the dummy cells 61 ₁ to 61 ₈ to the reference cell 41 ₂, as shown in FIG. 8. In FIG. 8, the left portion is a plan view of the reference cell array, and the right portion is a cross-sectional view taken along the line B-B in the plan view. Here, the circuit diagram corresponding to the reference cell array shown in FIGS. 7 and 8 is as shown in FIG. 9.

The foregoing embodiment offers the following advantageous effects. In this embodiment, the impurity is implanted into the dummy cells 61 ₁ to 61 ₈ so that the impurity exudes out of the dummy cell 61 ₁ to 61 ₈ to the reference cell 41 ₂. This provokes leak from the reference cell 41 ₂, under a high temperature. The leak lowers the voltage level of the reference cell 41 ₂, thereby preventing the voltage of the memory cell and that of the reference cell from being reversed, even when the memory cell incurs leak.

Such aspect will be further described referring to FIG. 3. With the start of the data read-out, the voltage V_(D1) and the voltage V_(R) respectively applied to the first and the second input terminal of the sense amp 23 ₁ increase through a generally similar process, until the selection cells 12 ₁₁, 12 ₂₁, the selection cells 42 ₁₁, 42 ₂₁ and the selection cells 42 ₁₂, 42 ₂₂ are turned ON. Then, also through a period until a H-level signal is applied to a location other than the word line 13, the voltage V_(D1) and the voltage V_(R) increase through a similar process (point A in FIG. 3).

When the H-level signal is applied to a location other than the word line 53, the voltage V_(R) incurs a drop in potential at a node F (FIG. 2) because of the leak from the reference cell 41 ₂, and resultantly moves as indicated by the line G in FIG. 3. The voltage V_(Dn), in contrast, descends as the line C in FIG. 3 when the memory cell 11 ₁ is an ON cell. When the memory cell 11 ₁ is an OFF cell and does not incur cell leak, the voltage V_(Dn) keeps increasing further as the line D in FIG. 3, however when the memory cell 11 ₁ is an OFF cell and incurs cell leak, the voltage V_(Dn) moves as shown by the line E in FIG. 3. Accordingly, even when the memory cell is an OFF cell and incurs cell leak, the level of the voltage V_(Dn) and the V_(R) is not reversed, and hence a correct decision is output. In addition, when the temperature is not high the leak current does not emerge, and hence neither the voltage V_(Dn) nor V_(R) is decreased, which assures stable performance of the storage device.

Also, as already stated, the second input terminal of the sense amp 23 ₁ receives the voltage V_(R), which is the median value between the voltage V_(RON) according to the ON state of the reference cell 41 ₁ and the voltage V_(ROFF) according to the OFF state of the reference cell 41 ₂, and therefore the voltage V_(R) is automatically set at the median value between the voltage V_(DON) based on an ON state of the memory cell 11 ₁ and the voltage V_(DOFF) based on an OFF state of the memory cell 11 ₁ constantly. As a result, a sufficient margin for detecting both of the ON state and the OFF state of the memory cell 11 ₁ can be secured.

The semiconductor storage device and the method of manufacturing the same according to the present invention are not limited to the foregoing embodiment, but various modifications may be made. To cite a few examples, the method of manufacturing according to the embodiment may include forming the word line connected to the reference cell 41 ₂ set in an OFF state in a finer size than the word line connected to the memory cell 11 ₁. Such configuration makes the effective L size between the drain and the source of the selection reference cell 41 ₂ finer, thereby allowing the impurity to exude upon performing the ion implantation. This assures emergence of the cell leak under a high temperature.

Also, when performing the impurity implantation, the impurity may be implanted at a higher dosage into the dummy cells 61 ₁ to 61 ₈ than into a memory cell set in an ON state among the memory cells. Such arrangement also allows the impurity to exude out of the dummy cells 61 ₁ to 61 ₈ to the selection reference cell 41 ₂, thereby assuring emergence of the cell leak under a high temperature.

It is apparent that the present invention is not limited to the above embodiment, and may be modified and changed without departing from the scope and spirit of the invention. 

1. A method of manufacturing a semiconductor storage device that compares a voltage between memory cells and reference cells to thereby read out data stored in said memory cells, comprising: forming a dummy cell at a position adjacent to a reference cell out of said reference cells that is set in an OFF state; and implanting an impurity into said dummy cell using a mask that covers said reference cell set in an OFF state; wherein said impurity is implanted so as to exude out of said dummy cell to said reference cell set in an OFF state.
 2. The method according to claim 1, wherein an opening of said mask corresponding to said dummy cell is larger in area than an opening of said mask corresponding to a memory cell out of said memory cells that is set in an ON state.
 3. The method according to claim 1, comprising forming a word line connected to said reference cell set in an OFF state so that said word line is finer than a word line connected to said memory cells.
 4. The method according to claim 1, wherein said impurity is implanted into said dummy cell at a higher dosage than into a memory cell out of said memory cells that is set in an ON state.
 5. A semiconductor storage device comprising: a reference cell that includes a first region located between two isolation regions and having a first impurity concentration, and a second region located between said first region and at least one of said isolation regions and having a second impurity concentration higher than said first impurity concentration.
 6. The semiconductor storage device according to claim 5, further comprising a dummy cell located on the respective sides of said reference cell; wherein an impurity concentration of said dummy cell is equal to or higher than said second impurity concentration.
 7. A semiconductor storage device that reads out information stored in a memory cell through comparison with information of a reference cell serving as a standard, wherein said memory cell has a first off-leak characteristic; and said reference cell has a second off-leak characteristic larger than said first off-leak characteristic. 